Totally blind people have sleep problems that are caused by desynchronized circadian rhythms. Without photic time cues, the internal body clock tends to "free-run" on about a 24.5 cycle; as a result, circadian rhythms move in and out of phase with desired sleep times, causing recurrent insomnia and impaired daytime alertness. Melatonin has been designated as an orphan drug for the treatment of circadian rhythm sleep disorders in blind people with no light perception. Nightly oral administration of melatonin may act as a "darkness signal" that can synchronize internal rhythms to desired sleep times. In addition to this "clock resetting" (phase shifting) action, melatonin may benefit insomnia in blind patients by a direct sleep-promoting (sedative) action.